Now that you have the Many to Many relationship created, you can navigate to the Record View of an account record, scroll down to the Account subpanel, then link an existing record. This will tie the two account records together via this subpanel. Let me know if you have any other questions!

If you want it to be a field on the record, you will need to create a One-to-Many or One-to-One relationship instead of a Many-to-Many relationship. Our documentation has more information on the different types of relationships.